# = lim_ (h +> 0) (x + h) ^ 2 + 9 (x + h) - 3 - (x ^ 2 + 9x -
# = lim_ (h 0) (x ^ 2 + 2xh + h ^ 2 + 9x + 9h - 3 - x ^ 2 - 9x + 3) / h #
취소 (x ^ 2) + 취소 (x ^ 2) + 취소 (9x) + 취소 (3x) + 취소)) / h #
# = lim_ (h-> 0) (2xh + h ^ 2 + 9h) / h #
# = lim_ (h-> 0) (h (2x + h + 9)) / h #
# = lim_ (h-> 0) (취소 (h) (2x + h + 9)) / 취소 (h) #
# = lim_ (h-> 0) 2x + 0 + 9 #
= 2x + 9
